1. Field of the Invention
The invention generally relates to a compact electronic display system. More specifically, the invention relates to a compact electronic display system which provides a compound magnified virtual image of a source object using two stages of magnification.
2. Description of Related Art
A continuing objective in the field of electronics is the miniaturization of electronic devices. Most electronic devices include some form of display system which provides information to the user. As electronic devices are reduced in size, display systems are needed which can be incorporated into the increasingly smaller devices. It is thus important that the space required to house these display systems be reduced. In particular, it is desirable that the thickness of the display system be reduced, the thickness of the display referring to the dimension of the display system which is perpendicular to the plane of the image formed by the display.
In general, the image provided by an electronic display may be either a real image or a virtual image. One approach to reducing the size of a display system is through the formation of a virtual image instead of a real image.
A real image refers to an image which is observed directly by the unaided human eye. A real image exists at a given location when a real image can be observed by the unaided eye if a viewing surface is positioned at the location. A photograph is an example of a real image. Examples of electronic displays which provide real images include liquid crystal displays, CRT monitors, and projection screens. Compact electronic devices, because of their small size, have a limited surface area on which to provide a real image. Since the amount of detail that the human eye can resolve per unit area is limited, devices which provide a real image are only able to provide a limited amount of legible information per display screen.
By contrast to real image displays, virtual image displays provide a virtual image, i.e., an image which, if a viewing surface were positioned at the location of the virtual image, no image would be observed by the eye. By definition, a virtual image can exist at a location where no display surface exists. An example of a virtual image is the image of fine print viewed through a magnifying glass.
Virtual image displays provide an image which appears to be larger than the source object from which the virtual image is formed. As a result, the size of the virtual image, as perceived by the user, is limited by the magnification of the display system as opposed to the size of the electronic display. This enables virtual image displays to provide the user with a greater amount of legible information per display screen than real image displays in the same space. It also enables a virtual image display to be designed which provides the same amount of information per screen as real image displays in a smaller space.
In general, virtual image displays include a source object which is magnified by one or more optics to provide a virtual image along an image plane. The thickness of the display, i.e., the dimension of the display that is perpendicular to the image plane of the virtual image, is dependent on the separation between the components of the display system. A need exists for an inexpensive, compact virtual image display system in which the separation between the components of the display system are reduced so that the display system has a reduced thickness.
There are at least four parameters which relate to the ease of viewing an image produced by a virtual image display. The first parameter is the far point which refers to the maximum distance from the eye that a display system can be held and have the eye still see the entire virtual image. Display systems which provide a far point which is a short distance from the display are undesirable due to the inconvenience and discomfort associated with placing the eye in close proximity with the display. A need therefore exists for a virtual image display system which provides a sufficiently long far point so that the magnified image can be viewed at a comfortable and convenient range of distances from the display system.
The second parameter relating to the ease of viewing a virtual image is the apparent angular width of the virtual image, commonly referred to as the field of view of the virtual image. The full field of view is defined as the ratio of the largest apparent dimension of the virtual image to the apparent distance to the virtual image. It is generally equivalent to the field of view for a real image display surface. A need exists for a virtual image display system which provides a wide field of view.
The third parameter relating to the ease of viewing a virtual image is the transverse distance that the eye may move with respect to the optical system and still have the eye see the entire virtual image through the display system. A need exists for a virtual image display system which provides a long transverse distance through which the eye may move with respect to the display system.
The fourth parameter relating to the ease of viewing a virtual image is illumination. In this regard, it is important that the virtual image produced be have a strong contrast ratio between illuminated and non-illuminated pixels. A need therefore exists for a display system which provides a bright virtual image. A further difficulty associated with virtual image displays is irregularities in the illumination of the source object. A need therefore also exists for a display system which provides a virtual image having substantially uniform illumination across the image.